PROJECT SUMMARY Complete understanding of brain function requires reliable and comprehensive mapping of large-scale brain networks with high spatiotemporal resolution and minimum invasiveness. Tools to achieve such mapping must overcome a myriad of challenges that are not adequately or simultaneously addressed by any existing technology. Hence the overall goal of this proposal is to develop a new diamond-based neural interface system that consists of up to 256 recording sites in mm3-sized volumes for combined electrical and chemical detection of neuronal activity in living nerve tissues. The proposed innovative tool will have the following significant advantages over existing technologies. First, highly-conductive BDD electrodes will simultaneously enhance the sensitivity, selectivity, and stability of neurological sensing. They will also have a greater potential range of operation than current electrode materials. Second, by using undoped PCD as a hermetic, biocompatible, and low-fouling encapsulation material, the new device will potentially have greater longevity and long-term stability for chronic applications. Third, a compact, dual-mode headstage will better enable the control of electrophysiology and fast-scan cyclic voltammetric (FSCV) measurements with high precision and a strong signal-to-noise ratio, while minimizing crosstalk. Fourth, the novel micromachining technique will permit wafer-level, mass production of diamond electrodes with various geometries, fine spatial resolution (submicrometer to micrometer scale), and high yields (>90%). Adopted from well-established semiconductor manufacturing techniques, the proposed fabrication approach is more reliable, consistent, scalable, and labor/cost-efficient than the hand assembly approach that is widely used today for making carbon fiber electrodes. Last but not least, 3D arrays of highly packed electrodes will significantly enhance the lateral and depth coverage of the new electrochemical detection tools compared to current chemical sensing tools. The project will be conducted by a multidisciplinary, collaborative team of researchers. The team will leverage their extensive experience in developing diamond fiber electrodes and in refining material synthesis and device fabrication techniques to push the spatial resolution of diamond electrodes from several tens of microns to submicrometer (via electron-beam lithography) and to micrometer (via ultraviolet lithography) (Aim 1). In parallel with electrode development, the team will engineer solutions to implement miniaturized head-mounted electrophysiology and FSCV electronics, and integrate the headstage with diamond electrode arrays to achieve a complete system (Aim 2). The functionality, biocompatibility, and stability of the integrated system will then be assessed ex vivo and in vivo using complementary analysis techniques (Aim 3). The proposed work is significant because it will yield a revolutionary neural interface tool that can be readily disseminated to other researchers for use in neuroscience and clinical studies to reveal the mechanisms underlying many brain disorders and diseases.